<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js"></script>
    <script>
        $(function() {
            $("button").click(function() {
                //$(":checkbox:eq(4)").attr("checked", true);
                $(":checkbox:eq(4)").prop("checked", true);
            })
        });
    </script>
    <style>

    </style>
</head>

<body>
    <select id="aaa">
        <option>1号</option>
        <option>2号</option>
        <option>3号</option>
        <option>4号</option>
        <option>5号</option>
        <option>6号</option>
        <option>7号</option>
    </select>

    <select id="bbb" multiple="multiple" SIZE="10">
        <option>1号</option>
        <option>2号</option>
        <option>3号</option>
        <option>4号</option>
        <option>5号</option>
        <option>6号</option>
        <option>7号</option>
    </select>
    <hr />
    <input type="checkbox" name="aaa">1号</input>
    <input type="checkbox" name="aaa">2号</input>
    <input type="checkbox" name="aaa">3号</input>
    <input type="checkbox" name="aaa">4号</input>
    <input type="checkbox" name="aaa">5号</input>
    <input type="checkbox" name="aaa">6号</input>
    <hr />
    <button>选中5号</button>
</body>

</html>